Hydraulic line pressure is used to engage clutches to select gear ratios in automatic transmissions or other types of hydraulically controlled gear boxes. Line pressure is controlled to avoid clutch slippage and to minimize energy consumption resulting from maintaining excessive line pressure. Controlling the line pressure of an automatic transmission affects fuel economy, the ability to control the transmission, and drive quality.
Ideally, the line pressure should be maintained at a level that is sufficient to sustain the holding capacity of the engaged clutches. The hydraulic pump may waste energy if the hydraulic line pressure is excessive. In addition, excessive line pressure may make clutch valve control more difficult due to the higher gain caused by the excessive line pressure. If line pressure drops to less than the level necessary to prevent clutch slippage, disturbances in the driveline may occur. Clutch slippage may also lead to thermal damage to the clutch lining.
In prior production transmissions, input shaft torque is estimated that causes some uncertainty and inaccuracy in controlling line pressure. U.S. Pat. Nos. 5,400,678 and 6,685,597 were considered in conjunction with the preparation of this patent application.
This disclosure is directed to solving the above problems and other problems that will be apparent in view of the following summary of the disclosure.